Optical modulation control device and mach-zehnder interference device

Abstract

An optical modulation control device includes: a photodetector or a photodetector which detects light emitted from a Mach-Zehnder interferometer and outputs an intensity signal indicating intensity of the light; and a phase-bias search unit which searches for and obtains a phase bias when the intensity signal outputted from the photodetector has a local minimum value or a phase bias when the intensity signal outputted from the photodetector has a local maximum value while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and records a set of the obtained phase bias and a wavelength of the light.

What is claimed is: 
     
         1 . An optical modulation control device, comprising:
 a photodetector to detect light emitted from a Mach-Zehnder interferometer and output an intensity signal that indicates intensity of the light; and   a phase-bias searcher to search for and obtain a phase bias when the intensity signal outputted from the photodetector has a local minimum value or a phase bias when the intensity signal has a local maximum value, while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and record a set of the obtained phase bias and a wavelength of the light,   wherein the phase-bias searcher comprises:   a phase-bias adjuster to adjust a phase bias injected into the optical path inside the Mach-Zehnder interferometer;   a delayer to keep the intensity signal outputted from the photodetector for a delay time and then output the intensity signal;   an amplifier to amplify the intensity signal outputted from the photodetector and output the intensity signal amplified;   a comparator to output a differential signal indicating a difference between the intensity signal outputted from the delayer and the intensity signal outputted from the amplifier; and   a phase-bias recorder to search for and obtain one or more phase biases when an absolute value of the differential signal outputted from the comparator is smaller than a threshold from among the phase biases injected into the optical path, search for and obtain a smallest intensity signal or a largest intensity signal among intensity signals for the obtained one or more phase biases among the intensity signals outputted from the photodetector, and record a set of a phase bias for the obtained smallest intensity signal and a wavelength of the light or a set of a phase bias for the obtained largest intensity signal and the wavelength of the light.   
     
     
         2 . The optical modulation control device according to  claim 1 , wherein the phase-bias adjuster adjusts an amplification factor of the intensity signal in the amplifier in accordance with the differential signal outputted from the comparator. 
     
     
         3 . The optical modulation control device according to  claim 1 , wherein the Mach-Zehnder interferometer divides incident light into two beams of light and emits composite light of the two beams of the light to the photodetector,
 the Mach-Zehnder interferometer has two optical paths as the inside paths through which the respective two beams of the light are transmitted, and   the phase-bias adjuster adjusts, in accordance with the differential signal outputted from the comparator, a phase bias injected into one of the two optical paths.   
     
     
         4 . The optical modulation control device according to  claim 1 , wherein the Mach-Zehnder interferometer divides incident light into two beams of light and emits composite light of the two beams of the light to the photodetector;
 the Mach-Zehnder interferometer has two optical paths as the inside paths through which the respective two beams of the light are transmitted;   the phase-bias adjuster adjusts, in accordance with the differential signal outputted from the comparator, a phase bias injected into each of the two optical paths.   
     
     
         5 . The optical modulation control device according to  claim 1 , wherein the Mach-Zehnder interferometer has a first output port to emit light and a second output port to emit light having a phase opposite to that of the light emitted from the first output port;
 the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local minimum value if the light detected by the photodetector is the light emitted from the first output port; and   the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local maximum value if the light detected by the photodetector is the light emitted from the second output port.   
     
     
         6 . The optical modulation control device according to  claim 2 , wherein the Mach-Zehnder interferometer has a first output port to emit light and a second output port to emit light having a phase opposite to that of the light emitted from the first output port;
 the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local minimum value if the light detected by the photodetector is the light emitted from the first output port; and   the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local maximum value if the light detected by the photodetector is the light emitted from the second output port.   
     
     
         7 . The optical modulation control device according to  claim 3 , wherein the Mach-Zehnder interferometer has a first output port to emit light and a second output port to emit light having a phase opposite to that of the light emitted from the first output port;
 the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local minimum value if the light detected by the photodetector is the light emitted from the first output port; and   the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local maximum value if the light detected by the photodetector is the light emitted from the second output port.   
     
     
         8 . The optical modulation control device according to  claim 4 , wherein the Mach-Zehnder interferometer has a first output port to emit light and a second output port to emit light having a phase opposite to that of the light emitted from the first output port;
 the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local minimum value if the light detected by the photodetector is the light emitted from the first output port; and   the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has the local maximum value if the light detected by the photodetector is the light emitted from the second output port.   
     
     
         9 . The optical modulation control device according to  claim 1 , wherein the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has a local minimum value, a phase bias when the intensity signal has a local maximum value while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and records a set of the phase bias when the intensity signal has the local minimum value, the phase bias when the intensity signal has the local maximum value and the wavelength of the light. 
     
     
         10 . The optical modulation control device according to  claim 2 , wherein the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has a local minimum value, a phase bias when the intensity signal has a local maximum value while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and records a set of the phase bias when the intensity signal has the local minimum value, the phase bias when the intensity signal has the local maximum value and the wavelength of the light. 
     
     
         11 . The optical modulation control device according to  claim 3 , wherein the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has a local minimum value, a phase bias when the intensity signal has a local maximum value while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and records a set of the phase bias when the intensity signal has the local minimum value, the phase bias when the intensity signal has the local maximum value and the wavelength of the light. 
     
     
         12 . The optical modulation control device according to  claim 4 , wherein the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has a local minimum value, a phase bias when the intensity signal has a local maximum value while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and records a set of the phase bias when the intensity signal has the local minimum value, the phase bias when the intensity signal has the local maximum value and the wavelength of the light. 
     
     
         13 . The optical modulation control device according to  claim 5 , wherein the phase-bias searcher searches for and obtains a phase bias when the intensity signal outputted from the photodetector has a local minimum value, a phase bias when the intensity signal has a local maximum value while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and records a set of the phase bias when the intensity signal has the local minimum value, the phase bias when the intensity signal has the local maximum value and the wavelength of the light. 
     
     
         14 . An optical modulation control device, comprising:
 a photodetector to detect light emitted from a Mach-Zehnder interferometer and output an intensity signal that indicates intensity of the light; and   a phase-bias searcher to search for and obtain a phase bias when the intensity signal outputted from the photodetector has a local minimum value or a phase bias when the intensity signal has a local maximum value, while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and record a set of the obtained phase bias and a wavelength of the light,   wherein the Mach-Zehnder interferometer comprises:   a first Mach-Zehnder interferometer having two optical paths which divide incident light into two light beams and transmit the respective two light beams;   a second Mach-Zehnder interferometer inserted into one optical path of the two optical paths of the first Mach-Zehnder interferometer; and   a third Mach-Zehnder interferometer inserted into the other optical path of the two optical paths of the first Mach-Zehnder interferometer,   the photodetector detects the light emitted from each of the first Mach-Zehnder interferometer, the second Mach-Zehnder interferometer and the third Mach-Zehnder interferometer,   the phase-bias searcher searches for and obtains a phase bias when a second intensity signal indicating intensity of the light emitted from the second Mach-Zehnder interferometer has the local minimum value or a phase bias when the second intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting a phase bias injected into an optical path inside the second Mach-Zehnder interferometer, and records a set of a wavelength of the light and the phase bias when the second intensity signal has the local minimum value or the local maximum value,   the phase-bias searcher searches for and obtains a phase bias when a third intensity signal indicating intensity of light emitted from the third Mach-Zehnder interferometer has the local minimum value or a phase bias when the third intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting a phase bias injected into the optical path inside the third Mach-Zehnder interferometer, and records a set of a wavelength of the light and the phase bias when the third intensity signal has the local minimum value or the local maximum value, and   the phase-bias searcher searches for and obtains a phase bias which is half a sum of a phase bias when a first intensity signal indicating intensity of light emitted from the first Mach-Zehnder interferometer has the local minimum value and a phase bias when the first intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting the phase bias injected into the optical path inside the first Mach-Zehnder interferometer, and records a set of a wavelength of the light and half the phase biases.   
     
     
         15 . An optical modulation control device, comprising:
 a photodetector to detect light emitted from a Mach-Zehnder interferometer and output an intensity signal that indicates intensity of the light; and   a phase-bias searcher to search for and obtain a phase bias when the intensity signal outputted from the photodetector has a local minimum value or a phase bias when the intensity signal has a local maximum value, while adjusting a phase bias injected into an optical path inside the Mach-Zehnder interferometer, and record a set of the obtained phase bias and a wavelength of the light,   wherein the Mach-Zehnder interferometer comprises:   a first Mach-Zehnder interferometer having two optical paths which divide a first polarized wave of incident light into two light beams and transmit the respective two first polarized light beams;   a second Mach-Zehnder interferometer inserted into one optical path of the two optical paths of the first Mach-Zehnder interferometer;   a third Mach-Zehnder interferometer inserted into the other optical path of the two optical paths of the first Mach-Zehnder interferometer;   a fourth Mach-Zehnder interferometer having two optical paths which divide a second polarized wave of incident light into two light beams and transmit the respective two second polarized light beams;   a fifth Mach-Zehnder interferometer inserted into one optical path of the two optical paths of the fourth Mach-Zehnder interferometer; and   a sixth Mach-Zehnder interferometer inserted into the other optical path of the two optical paths of the fourth Mach-Zehnder interferometer;   the photodetector detects the light emitted from each of the first Mach-Zehnder interferometer, the second Mach-Zehnder interferometer, the third Mach-Zehnder interferometer, the fourth Mach-Zehnder interferometer, the fifth Mach-Zehnder interferometer and the sixth Mach-Zehnder interferometer,   the phase-bias searcher searches for and obtains a phase bias when a second intensity signal indicating intensity of light emitted from the second Mach-Zehnder interferometer has the local minimum value or a phase bias when the second intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting a phase bias injected into the optical path inside the second Mach-Zehnder interferometer, and records a set of a wavelength of the light and the phase bias when the second intensity signal has the local minimum value or the local maximum value,   the phase-bias searcher searches for and obtains a phase bias when a third intensity signal indicating intensity of light emitted from the third Mach-Zehnder interferometer has the local minimum value or a phase bias when the third intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting phase bias injected into the optical path inside the third Mach-Zehnder interferometer, and records a set of a wavelength of the light and the phase bias when the third intensity signal has the local minimum value or the local maximum value,   the phase-bias searcher searches for and obtains a phase bias which is half a sum of a phase bias when a first intensity signal indicating intensity of light emitted from the first Mach-Zehnder interferometer has the local minimum value and a phase bias when the first intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting a phase bias injected into the optical path inside the first Mach-Zehnder interferometer, and records a set of a wavelength of the light and half the phase biases,   the phase-bias searcher searches for and obtains a phase bias when a fifth intensity signal indicating intensity of light emitted from the fifth Mach-Zehnder interferometer has the local minimum value or a phase bias when the fifth intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting a phase bias injected into the optical path inside the fifth Mach-Zehnder interferometer, and records a set of a wavelength of the light and the phase bias when the fifth intensity signal has the local minimum value or the local maximum value,   the phase-bias searcher searches for and obtains a phase bias when a sixth intensity signal indicating intensity of light emitted from the sixth Mach-Zehnder interferometer has the local minimum value or a phase bias when the sixth intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting a phase bias injected into the optical path inside the sixth Mach-Zehnder interferometer, and records a set of a wavelength of the light and the phase bias when the sixth intensity signal has the local minimum value or the local maximum value, and   the phase-bias searcher searches for and obtains a phase bias which is half a sum of a phase bias when a fourth intensity signal indicating intensity of light emitted from the fourth Mach-Zehnder interferometer has the local minimum value and a phase bias when the fourth intensity signal has the local maximum value from among intensity signals outputted from the photodetector while adjusting phase bias injected into the optical path inside the fourth Mach-Zehnder interferometer, and records a set of a wavelength of the light and half the phase biases.   
     
     
         16 . A Mach-Zehnder interference device, comprising:
 a Mach-Zehnder interferometer having an optical path which divides incident light into two light beams and transmits two divided light beams;   a photodetector to detect light emitted from the Mach-Zehnder interferometer and outputting an intensity signal that indicates intensity of the light; and   a phase-bias searcher to search for and obtain a phase bias when the intensity signal outputted from the photodetector has a local minimum value or a phase bias when the intensity signal has a local maximum value, while adjusting a phase bias injected into the optical path of the Mach-Zehnder interferometer, and record a set of the obtained phase bias and a wavelength of the light,   wherein the phase-bias searcher comprises:   a phase-bias adjuster to adjust a phase bias injected into the optical path inside the Mach-Zehnder interferometer;   a delayer to keep the intensity signal outputted from the photodetector for a delay time and then output the intensity signal;   an amplifier to amplify the intensity signal outputted from the photodetector and output the intensity signal amplified;   a comparator to output a differential signal indicating a difference between the intensity signal outputted from the delayer and the intensity signal outputted from the amplifier; and   a phase-bias recorder to search for and obtain one or more phase biases when an absolute value of the differential signal outputted from the comparator is smaller than a threshold from among the phase biases injected into the optical path, search for and obtain a smallest intensity signal or a largest intensity signal among intensity signals for the obtained one or more phase biases among the intensity signals outputted from the photodetector, and record a set of a phase bias for the obtained smallest intensity signal and a wavelength of the light or a set of a phase bias for the obtained largest intensity signal and the wavelength of the light.